In a wireless transmitter or a wireless receiver, a mixer is widely used as a frequency conversion device. FIG. 1 shows a conventional wireless transmitter 10 capable of converting a base-band transmitting signal to a radio frequency (RF) transmitting signal to be transmitted via an antenna. The wireless transmitter 10 comprises filters 11 and 12, programmable gain amplifiers 13 and 14, mixers 15 and 16, and a power amplifier 17. After removing needless frequency components from a base-band transmitting signal I by the filter 11, the base-band transmitting signal I is amplified by the programmable gain amplifier 13 and then sent to the mixer 15, where the base-band transmitting signal I is converted to an RF I signal according to a local oscillator signal LOI generated by a local oscillator (not shown). A base-band transmitting signal Q is similarly converted to an RF Q signal, which is sent to the power amplifier 17 together with the RF I signal, so as to amplify the RF I and Q signals for wireless transmission.
FIG. 2 is a conventional wireless receiver 20 capable of converting an RF receiving signal to a base-band receiving signal for subsequent signal processes. The wireless receiver 20 comprises a low noise amplifier 21, mixers 22 and 23, filters 24 and 25, and programmable gain amplifiers 26 and 27. After being amplified by the low noise amplifier 21, frequencies of in-phase and quadrature-phase signals of the RF receiving signal are converted into base-band frequencies respectively by the mixers 22 and 23 according to local oscillator signals LOI and LOQ generated by a local oscillator (not shown). After removing needless frequency components by the filters 24 and 25 and amplifying by the programmable gain amplifiers 26 and 27, base-band receiving signals I and Q are generated. Hence, signal quality of the wireless communication depends largely on the frequency conversion of the mixer 15 and 16, 22 and 23.
FIG. 3 is a circuit diagram of a conventional mixer. Referring to FIG. 3, a Gilbert mixer 30 comprises a transconductor circuit 31, a switch circuit 32 and a load circuit 33 having loads 331 and 332. Each of the loads 331 and 332 has its one end coupled to a voltage source Vcc and its other end serving as an output end. The switch circuit 32 comprises n-channel transistors M3, M4, M5 and M6. The transistors M3 and M5 have their drains coupled to one end of the load 331, and the transistors M4 and M6 have their drains coupled to one end of the load 332. Moreover, the transistor M3 and the transistor M6 have their gates coupled to each other, the transistor M4 and the transistor M5 have their gates coupled to each other. The gates of the transistors M3 and M4 are capable of receiving a local oscillator signal LO. The transistor M3 and the transistor M4 are coupled to each other to form a first current path, and the transistor M5 and the transistor M6 are coupled to each other to form a second current path.
The transconductor circuit 31 comprises n-channel transistors M1 and M2. The transistor M1 has its drain coupled to the first current path of the switch circuit 32, and the transistor M2 has its drain coupled to the second current path of the switch circuit 32. Gates of the transistors M1 and M2 respectively receive differential voltage signals Vin+ and Vin−. The sources of the transistors M1 and M2 are coupled to each other. Moreover, an n-channel transistor MS is coupled between the source of the transistor M1 and a ground terminal; and a fixed voltage is inputted into a gate of the n-channel transistor MS such that the n-channel transistor MS forms a current source.
FIG. 4 is a schematic diagram of signals associated with the Gilbert mixer 30. The transconductor circuit 31 converts input differential voltage signals such as the Vin+ and Vin− to a current signal Ib. When flowing through the first current path and the second current path of the switch circuit 32, the current signal Ib, being driving by an oscillator signal LO, becomes a frequency-converted current signal, e.g., the current signal Ib is converted from a base-band frequency to a radio frequency as illustrated in FIG. 4. After that, the frequency-converted current signal is converted by the load circuit 33 so that an output voltage is generated at the output end.
However, transistors of a conventional mixer are not completely ideal. For example, the transistors have nonlinear characteristics, due to which harmonic interferences are generated at an output voltage of the mixer, and thus signal quality of a frequency conversion is reduced.
In addition, in the conventional mixer, bias points of a transconductor circuit and a switch circuit are correlative rather than being independent so that linearity of the transconductor circuit is influenced. For example, in the Gilbert mixer 30 illustrated in FIG. 3, correlation exists between bias points of the transconductor circuit 31 and the switch circuit 32. When the bias point of the switch circuit 32 is too low, the bias point of the transconductor circuit 31 becomes too low such that the transistors M1 and M2 can not operate in a saturation region and the linearity of the transconductor circuit 31 is swayed. When the bias point of the transconductor circuit 31 or the switch circuit 32 is shifted higher in order to avoid the foregoing problem, the switch circuit 32 may not operate normally.